WANA (APP) - Five children were killed and two injured when an undefused mortar shell exploded in a house at Azam Worsk here in South Waziristan Agency on Thursday. According to sources in political administration, the children of Malik Zayid Ullah Wazir were playing with an undefused bomb brought by them from nearby fields, suddenly went-off. As result of which five children died instantly while two others sustained injuries. The injured were shifted to the Agency's Headquarters hospital, Wana. Meanwhile, at least four extremists were killed when a clash broke out between the security forces and miscreants in the night between Wednesday and Thursday in Bara Tehsil, official sources told APP. An official of the FC got injured in the incident. Separately, security personnel arrested seven extremists including a Taliban commander from Shah Kas area of the Agency. Meanwhile, a man was killed and one of his rivals sustained bullet injuries in a crossfire in Paharkhel Thall village in the area limits of Lakki Marwat police station, police sources said on Thursday. The sources said that Qadir and his rivals Waris and Hanif exchanged firing when they confronted with each other in the outskirts of the village. "As a result Qadir was shot dead while his rival Waris received bullet wounds", sources said, adding that both parties had a blood feud. "Police have filed separate cases against the accused persons of both sides under relevant section of law and started investigation", sources added.